1. Field of the Invention
The present invention generally relates to electronics. In particular, the present invention relates to communications systems.
2. Description of the Related Art
The rapid commoditization of the cellular, personal communication service (PCS) and wireless industries has resulted in the emergence of new digital radio standards, which support the emergence of high user bandwidth requirements. For example, third generation (3G) digital wide-band code division multiple access (W-CDMA) and Enhanced Data GSM (Group System for Mobile Communications) Environment (EDGE) air interface standards exploit signal processing techniques that can generate radio and baseband waveforms with a relatively high peak power to average power ratio.
The signals amplified by a wireless base station include multiple signals, which are combined to a multi-bearer waveform. The number of voice and data connections represented within the multi-bearer waveform can vary randomly and vary over time. Occasionally, the information sources that are combined to form the multi-bearer waveform can co-align and generate a relatively large instantaneous signal peak or crest. In one example, the relatively large instantaneous signal peak is about 10 times higher in power than a nominal or average output level.
In practice, the alignment that generates a relatively large instantaneous signal peak occurs with a relatively low probability. Despite the relatively low probability, however, the dynamic range of the entire signal processing chain of a base station should be sufficient to handle the large instantaneous signal peak in order to transmit the signal without error.
One conventional approach is to design the base station to accommodate the relatively rare, but large, signal peak. As a result, the base station is significantly overdesigned, which results in a significant increase to the cost of the base station. In particular, the cost and the size of the radio frequency (RF) amplifier of the base station are deleteriously affected. For example, such an approach disadvantageously lowers the efficiency of the RF amplifier, as a higher powered RF amplifier will waste significantly larger amounts of power for biases and the like. Further, the extra power dissipation is correspondingly dissipated with larger and more costly heat management techniques.
In addition, the relatively large dynamic range imposed upon the base station by the relatively large signal peak typically requires that the upconversion circuitry, the digital to analog converters, the digital signal processing circuits, and the like also accommodate the relatively large dynamic range.
In another conventional approach, the signal waveform is hard limited to reduce the dynamic range of the relatively rare signal peaks. This allows a relatively lower power RF transmitter to be used to transmit the signal, which allows the RF transmitter to operate with relatively larger efficiency. However, conventional hard limiting techniques are impractical because hard limiting generates distortion energy, which causes interference in adjacent channels.